Re: Procedure types

From: [at]} <Wojtek>
Date: Sun, 21 Nov 2004 20:17:21 -0500 (EST)

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On Sun, 21 Nov 2004, Wojtek Skulski wrote:

> Inheritance references are static, while procedure variables are assigned
> at run time. Furthermore, even the number of procedure variables
> in a given module may not be known since procedure variables can be
> created dynamically. For example, imagine a type
>
> prVar = POINTER TO RECORD pv: PROCEDURE; next: prVar END;
>
> One can thus create a linked list of records, each one containing
> a reference to a procedure. In order to make unloading safe one needs
> to maintain those references versus unloading of modules that implement
> the "pv" fields.

On my second thought: the example above may lead to "dynamic imports" as
opposed to static imports that are known at compile time. My earlier
example with two modules TestUpper and TestLower could illustrate the
same idea. Namely, either a procedure variable or an object reference
from the module Upper gets installed in the hook variable declared in
module Lower, however, without statically importing the module Upper.
This means that at run time the module Lower contains references
to the module Upper. I would call it "dynamic import" because
the dependency was created at run time.

Now imagine Upper is unloaded. What happens if the hook variables are not
set to NIL but rather hold pointers to the unloaded code?

One solution might be that before a given module is unloaded, all
pointers in all other modules are traversed the same way as GC is doing
now, and in case a reference is encountered to the Upper code, it is not
unloaded but an error message is issued. The traversal should happen after
the CLOSE part of Upper is executed, because in all likelihood a sane
programmer has taken care of the problem by closing all references.
However, in order to prevent a dangling pointer still existing somewhere,
this last-minute check can be done. If Upper is still "dynamically
imported" from any place in the system, it is kept in memory.

I would not complain about the inefficiency of this solution, because if
we accept GC then we should also accept this. However, I am not sure if
this really solves the problem. I am also sure I am reinventing the wheel
here. This line of thought must be well known.


> I am sure one can invent even more vicious scenarios. My question still
> stands whether all this is safe in the current version of the run time
> system, and if not then whether it can be made safe. I am not yet buing
> into Fyodor arguments. Perhaps this problem has been solved already
> by the c.s. community. I simply do not know. I am only a user.

W.


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Received on Mon Nov 22 2004 - 02:17:21 UTC